Rendering is the process of generating an image from a model such as a three-dimensional terrain model by means of a computer programme. The three-dimensional terrain model to be rendered can be a grid containing elevation data, such as a Digital Elevation Model (DEM), a Digital Surface Model (DSM) or a Digital Terrain Model (DTM). The digital model may comprise data of a limited area, such as a model of a particular city or landscape, as well as a model of a continent or even a complete planet.
When rendering three-dimensional terrain data in a computer, it is important to partition the data in such a way that not all of the data needs to be stored in the memory of the device, thus reducing the amount of memory needed on the device. This is especially important for mobile devices, which generally have particularly limited memory capacities.
Computer rendering of three-dimensional terrain images is generally known from prior art: U.S. Pat. No. 7,551,172 B2 discloses a method for sending information representing three-dimensional images over a network, and U.S. Pat. No. 6,496,189 B1 discloses a method and apparatus for displaying images of an area as seen from an interactively chosen viewpoint on a remote device.
In a common method for rendering three-dimensional terrain data, a tree is used, which divides the data into tiles and levels of detail, wherein each level of detail comprises more information than the previous one, thus allowing zooming in or out while keeping the rendering quality and preserving the amount of memory needed on the device. Disadvantageously, if the tiles are displayed three-dimensionally, overlapping tiles on top of each other will appear, i.e. tiles that cover the same area, but have a different detail level. As these tiles do not comprise the exactly same amount of information, the tiles are not perfectly overlapping. This problem appears, because tiles of a higher detail level, i.e. tiles comprising more information, should be displayed on top of tiles of a lower detail level, i.e. tiles including less information. But due to overlapping, it may happen that some part of a tile of a lower detail level is displayed—partially or completely—on top of another tile of a higher detail level, leading to so-called artifacts.
Various methods to remove these artifacts are known from prior art. One method employs the use of a “clipping plane”. For avoiding overlapping, such a method comprises determining for each tile, which area of the tile should be rendered. Disadvantageously, this method is relatively slow, especially on mobile devices, since complex 3D calculus needs to take place as well as complex logic for determining the area of the tile that needs to be rendered.
Another method uses the “depth offset”. Therein, an offset is added to each tile using its level of detail value. The main problem of this approach is that it adds a “hovering” effect on all the tiles on top of each other and does not work if the amount of data on the data levels varies strongly.